Metal works (rolling process in particular), in which a high-frequency pulse oscillator of the present invention may be employed, will be discussed below. Rolling refers to a process that induces plastic deformation of metallic materials by compressing and squeezing the materials between work rolls. Rolling is a type of plastic processing wherein production speed is high and dimensional accuracy can be easily controlled. Since rolling requires relatively lower production costs and provides products having more uniform dimensional accuracy and quality than molding or casting, it is most frequently used. In a rolling process, materials to be processed receive compressive stress caused by the pressing force of the rolls while they pass through the rolls, and further undergo shear stress on the interfaces with the rolls. The shear stress acts to thrust the materials between the rolls.
There are essentially two types of rolling process, i.e., hot rolling and cold rolling. Cold rolling refers to rolling the materials to be processed, such as ingots, at a normal temperature and is used for providing products such as sheets, strips and foils, which have high strength and good dimensional accuracy. On the other hand, hot rolling refers to rolling metallic materials after applying heat with a heating furnace. Generally, through hot rolling, ingots are processed into blooms or billets and are further processed into boards, sheets, bars, pipes and rails.
Since cold rolling is performed at a normal temperature, it is advantageous in that there is no need for any special equipments for heating materials such as strips. However, cold rolling typically requires an annealing process. As such, the entire rolling process time becomes lengthened while the productivity is lowered.
In hot rolling, a metal strip is heated in a heating furnace and is then fed to a rolling device. It is extremely important to heat the metal strip to a specified temperature before rolling. That is, if the heating temperature is much lower than the specified temperature, then various problems may occur (e.g., difficulty in carrying out rolling, excessive loads are imposed on the rolling device, and desirable properties cannot be obtained for the rolled strip). However, if an excessively high temperature is set as the heating temperature (considering that temperature decreases during the transfer of the heated metal strip to the rolling device), then there is an increase in the oxidization of the metal strip. Thus, the resulting energy costs inevitably increase and may even become burdensome. Under these circumstances, it is advisable in hot rolling to heat the metal strips as closely as possible to the work rolls. For such purpose, it is plausible to employ a high-frequency induction heating method and an electric heating method.
However, a high-frequency induction heating device is usually complicated, expensive and consumes too much power.
On the other hand, in the electric heating method, the work rolls and the metal strip are heated by their respective electrical resistance when direct currents from a power source are applied to the upper/lower work rolls and the strip. Thus, their temperatures tend to rise. An example of an electric heating type rolling device is disclosed in Japanese Patent Publication No. 1998-180317. However, the prior art electric heating type rolling device consumes too much power. For instance, when a steel strip having a width of 100 mm and a thickness of 2 mm (i.e., a cross-sectional area of 2 cm2) is rolled to have a thickness of 0.25˜0.3 mm by electric-heating, a current density of about 104 A/cm2 is required. If a direct current is applied, then the current strength reaches 20 kA, which is a multiplication between the current density and the cross-sectional area of the strip to be rolled. In addition to such excessive power-consumption, the steel strip is heated to a temperature ranging from 400° C. to 500° C., which causes oxidization and discoloration on the strip surface. Further, since the work rolls are included in the electric circuit, the life of work rolls may be shortened due to electric corrosion. Also, a cooling device, which prevents the work rolls from being damaged due to heat transfer from the steel strip, is additionally required. Moreover, adverse effects upon the environment may occur around the production premises.